The invention relates to a hybrid energy system for vehicles, which hybrid energy system comprises an autonomous power supply and is connectable to an external power supply infrastructure along the route of said vehicle, and a method for operating the system.
In recent years, development and commercialization of electric and hybrid vehicles that are effective in reducing fuel consumption and exhaust gases such as CO2 have been pursued. Electric vehicles for commercial use have a limited range because there are limits to the size and capacity of batteries mounted in the vehicles.
For hybrid vehicles provided with an internal combustion engine, or ICE, combined with an electric motor, the supply of electric energy is also dependent on a battery mounted in the vehicle. Consequently the cruising range of such a vehicle in electric mode is limited because of the limitations on the size and capacity of batteries mounted in the vehicle. Hybrid vehicles must therefore use an engine and a motor in combination to secure a long cruising range.
On the other hand, the cruising ranges of rail bound vehicles using electric energy are not limited because they run on electrical power received from overhead wires or an electric rail adjacent the track. For such vehicles it is necessary to lay tracks and manage them, which requires high construction cost and high maintenance cost. Hence, they are not suitable for transportation of goods between arbitrary locations Also, when there is an obstacle on a track or if a power outage occurs, the vehicle will be delayed until the problem has been eliminated.
A solution to the above problems are vehicles running on electrical power received from overhead wires such as “trolley buses” which were in common use in many cities in the past. In the subsequent text such vehicles will be referred to as power-collecting electric vehicles. Although no tracks are needed for such vehicles, they do need overhead wires, and they cannot run on roads unequipped with overhead wires, due to limited battery capacity. Hence, they do not have flexibility of general commercial vehicles.
Technology has since been developed to enable such power-collecting electric vehicles to run on roads not equipped with overhead wires. Such vehicles are equipped with a motor for propulsion, driven by electrical power received from overhead wires, as well as an internal combustion engine for generating electrical power to run the drive motor, for supplying mechanical power directly to a drive unit of the vehicle, or for charging on-board batteries. Such vehicles have been developed by applying hybrid technology to the power-collecting electric vehicle. Such hybrid vehicles have been developed by, for instance, Siemens AG for an “e-Highway” concept within the framework of the EU Seventh Framework Programme.
One problem with hybrid power-collecting electric vehicles is that the vehicles comprise an electric circuit with components that operate at different voltages. The circuit comprises a high voltage power-collecting system, connectable to overhead wires, and a hybrid electric system, comprising power electronic components and, optionally, a high voltage battery. Such arrangements require the use of a power converter that allows the high voltage from the overhead lines to be utilized by the vehicle. The power converter is usually a DC/DC converter, which can handle relatively high voltage, e.g. 500-700 V in the overhead wires. According to one example, the hybrid vehicle can comprise a high voltage power-collecting system and a power converter connected to one or more electric motors in a hybrid propulsion system comprising an ICE connectable to an electric motor. For a hybrid vehicle not provided with a storage battery, the power converter must be dimensioned for a continuous rating equal to the peak power requirement of the propulsion system, that is, at least 200-300 kW. Such an arrangement would be used for a vehicle mainly operated using the ICE.
According to a further example, the hybrid vehicle can comprise a high voltage power-collecting system and a power converter connected to a storage battery and one or more electric motors in a hybrid propulsion system comprising an ICE connectable to an electric motor. For a hybrid vehicle provided with a storage battery, the power converter can be dimensioned for a continuous rating equal to the average power requirement of the propulsion system, that is, at least 100-150 kW. Such an arrangement would be used for a vehicle mainly operated using the power-collecting system, where the ICE can be used for charging the storage battery.
A power converter connected directly to the high voltage power-collecting system, as indicated in the above examples, requires all electric power to pass through the power converter. This incurs conversion losses and generates heat that requires cooling, which reduces the overall system efficiency and increases the demand on the vehicle cooling system. A power converter of this type would also be relatively large and expensive.
It is desirable to solve the above described problems and to provide a hybrid vehicle with a power collector and an operation system that can reduce the cost of manufacturing the vehicle. It is also desirable to provide a hybrid vehicle with an improved overall system efficiency and which incurs lower conversion losses and heat generation.
In the subsequent text the term “electric road system” or ERS will be used for a network of roads provided with means for supplying electric power to a vehicle. The examples below will be described for a case where the power supply comprises overhead wires. However, the invention is not limited to a conductive power supply, using an overhead electrified wire or track/rail in or adjacent the road surface as in the “Electric roads concept” proposed by Volvo Trucks, but can also use an inductive power supply. The invention will be described in relation to a commercial over the highway truck or tractor, but is equally applicable to off-highway trucks/tractors, buses, construction vehicle or other types of work vehicles.
The subsequent text will also refer to a number of different technical terms and examples of electrical components, which will be defined briefly below.
Voltage regulation is a measure of change in the voltage magnitude between the sending and receiving end of a component, such as a transmission or distribution line. Voltage regulation describes the ability of a system to provide near constant voltage over a wide range of load conditions. The term may refer to a passive property that results in more or less voltage drop under various load conditions, or to the active intervention with devices for the specific purpose of adjusting voltage.
Power conversion is converting electric energy from one form to another, converting between AC and DC, or just changing the voltage or frequency, or some combination of these. In this context, the general term “power converter” is defined as an electrical or electro-mechanical device for converting electrical energy. This could be as a transformer to change the voltage of AC power, but the term also refers to a class of electrical machinery that is used to convert one frequency of alternating current into another frequency. Power conversion systems often incorporate redundancy and voltage regulation. One way of classifying power conversion systems is according to whether the input and output are alternating current (AC) or direct current (DC).
One type of power converter is a DC-to-DC or DC/DC converter, which is an electronic circuit which converts a source of direct current (DC) from one voltage level to another. DC/DC converters employ existing technological designs, where the main topological classes are fixed frequency pulse width modulation (PWM) and variable frequency quasi-resonant zero current switching (ZCS).
PWM can be somewhat simpler in design, but it inherently trades off efficiency against operating frequency, both important parameters for electric vehicles (EV) or hybrids (HEV). High-frequency operation has long been recognized as one of the main keys to achieving high-power density, e.g. smaller magnetics, filters, and capacitors, in switch mode converters. With fixed frequency switch mode converters, however, switching losses increase directly with operating frequency, resulting in the right place which limits achievable power density. Variable-frequency converters overcome the frequency barrier by having each turn-on and turn-off of the switch occur at zero current.
A further difference between fixed frequency and variable frequency DC/DC converters is the noise. Again, an important parameter for EVs/HEVs is noise generated by the switch. The hard switching of the PWM generates more noise than the soft switching of ZCS.
Previously, the primary EV/HEV DC/DC converter application is the conversion from a high voltage battery down to the 12-volt typical car voltage, although higher voltages, such as 42 Volts for power steering, may be required. DC/DC converters used in this application typically have inputs of 250-450 Volts, adjustable outputs of 12.5 to 15.5 Volts, and an output power from 250 W to 3.5 kW. The sizes and weights of available DC/DC converters vary substantially, dependent on the operating frequency, of course, but also to some extent on the inputs and outputs of voltage and power. With conventional topologies, efficiencies are typically mid-80-90%, but the low line efficiencies are likely to be perhaps four or five percentage points lower. As a result, AC-DC and some wide-range DC/DC products need to be derated at the low line.
High-voltage/high-power conversion in vehicles is a preferred solution for EV and HEV applications. The technical challenges for such a converter, many of them interrelated, include size, weight, efficiency, electromagnetic compatibility/electromagnetic interference (EMC/EMI), reliability, high-voltage isolation, heat removal/thermal management, and, cost. In addition, reliable performance in the environments of heat, cold, shock, and vibration of a road vehicle is required.
DC/DC converters for commercial EVs and HEVs require high power density, efficiency, and a scalability that cannot be cost-effectively supported by low frequency, bulk converter designs. While a 2 kW DC/DC converter may be a common design target, high-end vehicles require more power, whereas smaller DC/DC converters with lower power ratings would provide lower cost for entry-level EVs and HEVs. To cope with this range of power needs, a flexible, scalable power system methodology using high-power density, modular converters capable of efficient bus conversion, isolation and voltage regulation will enable greater performance, cost-efficiency and a faster time-to-market.
Modern DC/DC power converters can support efficient high-voltage electric power distribution within vehicles and provide key advantages to the power system designer, including small size, low weight, high power density, high efficiency, design flexibility, and fast response to changing electrical demands. Specifically, DC/DC power converters particularly suited for EV/HEV vehicles include Zero-Voltage Switching (DC/ZVS) DC/DC converters with 95% efficiency at 1 kW/in3 power density; ZVS Buck-Boost regulators with >97% efficiency at 1 kW/in3; and Sine Amplitude Converter™ High Voltage (SAC HV) bus converters with 97% efficiency at 1 kW/in3.
Double clamp zero voltage switching (DC/ZVS) converters have the capability of providing a regulated output from a very wide input range. Adaptive cell power systems involve a multiplicity of converters that are configured in an array to provide wide-range, high-voltage, high-frequency power processing. A converter block typically utilizes two magnetically coupled converter cells that are selectively configured in series or parallel. In either configuration, common-mode noise is essentially cancelled, eliminating a major filtering challenge for EVs and HEVs.
Adaptive cell topologies embodied in DC/ZVS DC/DC converters for EV and HEV DC/DC converter performance may include Sine Amplitude Converter (SAC) cells. SAC engines utilize zero-voltage/zero-current switching to eliminate switching losses. By eliminating switching loss, the SAC can be operated efficiently at relatively high frequencies, typically in the MHz range, resulting in smaller product size. High operating frequency allows for miniaturization of many components, increasing overall converter power density. Soft switching converters operating at high frequency also minimize electromagnetic interference (EMI) and the filtering components required by hard-switching converters operating at low frequency.
The SAC engine is typically used to provide fixed voltage ratio bus conversion with HV isolation. The DC-ZVS engine provides DC/DC conversion with regulation and isolation.
ZVS buck-boost regulators provide a regulated output from an unregulated input source. ZVS buck-boost regulators may be used standalone, as non-isolated voltage regulators, or combined with SAC current multipliers to create isolated DC/DC converters. The regulator may be “factorized” away from SAC current multipliers to provide increased density at the point of load while supporting efficient power distribution and savings in conductor weight and cost. In combination, these engines enable DC/DC converter systems with significantly higher density, flexibility, and efficiency than conventional converters. ZVS buck-boost regulator capabilities include input and output voltages up to at least 650 Vdc and conversion efficiency up to 98%.
A unique soft switching topology and ZVS control architecture enable efficient HV operation at 1 MHz. Regulators may be paralleled to achieve increased output power. A feature of the regulator control architecture is that its switching sequence does not change in either buck or boost mode. Only the relative duration of phases within each operating cycle are controlled to effect voltage step up or step down.
Fixed-ratio converters, which include the SAC HV bus converter, are capable of efficient HV bus conversion. SAC HV bus converter capabilities include input and output voltages up to at least 650 Vdc and conversion efficiency up to 98%.
ZVS-ZCS Sine Amplitude Converter topologies with a low Q power train support efficient high frequency power processing with a fixed-frequency oscillator having a high spectral purity and common-mode symmetry, resulting in essentially noise-free operation. The control architecture locks the operating frequency to the power train resonant frequency, optimizing efficiency and minimizing output impedance. By effectively cancelling reactive components, output impedance, Zout, can be relatively low. To further reduce Zout, or for greater power capability, bus converters can be paralleled with accurate current sharing. Quiet and powerful, SAC bus converters provide essentially linear voltage/current conversion with flat output impedance up to about 1 MHz.
In combination, these solutions are examples of power converters well suited for commercial EVs and HEVs including small size, low weight, very high efficiency, low EMI, high-voltage isolation, heat management, modularity, design flexibility, scalability, and cost. They are easily paralleled to configure fault-tolerant high-power arrays.
Another type of power converter is a DC-to-AC, or DC/AC power converter, often termed inverter. This is an electrical power converter that changes direct current (DC) to alternating current (AC). The converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits. Solid-state inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high-voltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as overhead wires or batteries.
A variable-frequency drive (VFD) controls the operating speed of an AC motor by controlling the frequency and voltage of the power supplied to the motor. An inverter provides the controlled power. In most cases, the variable-frequency drive includes a rectifier so that DC power for the inverter can be provided from main AC power. AC power supplied from a motor operated as a generator can also be rectified for charging a battery. Since an inverter is the key component, variable-frequency drives are sometimes called inverter drives or just inverters. VFDs that operate directly from an AC source without first converting it to DC are called cycloconverters. They are now commonly used for driving traction motors.
Adjustable speed motor control inverters are currently used to power the traction motors in some electric and diesel-electric rail vehicles as well as some battery electric vehicles and hybrid electric highway vehicles. Various improvements in inverter technology are being developed specifically for electric vehicle applications. In vehicles with regenerative braking, the inverter also takes power from the motor acting as a generator and stores it in batteries or a similar suitable energy storage system.
According to a preferred embodiment, the invention relates to a hybrid energy system in a vehicle. The hybrid energy system comprises an autonomous power supply and is connectable to an external power supply infrastructure or grid along the route of said vehicle. The vehicle is arranged to operate in an autonomous power supply mode using an on-board energy storage system, in an external power supply mode using electrical power from overhead wires or a roadside rail, or in a combined autonomous and external power supply mode using electrical power from both sources.
According to an aspect of the invention, the hybrid energy system comprises a high voltage propulsion system split into two parts or high voltage circuits inside the vehicle. The hybrid energy system comprises a first high voltage circuit comprising a first traction motor connected to an energy storage system by a first power converter for propelling the vehicle. The hybrid energy system further comprises a second high voltage circuit comprising a second traction motor connectable to an external source of electrical power by a second power converter for propelling the vehicle. The first and the second traction motor can be operated as motors, for propelling the vehicle, or as generators, for regeneration of energy.
The first and the second traction motor can each be mechanically connected to an individual or a common ground engaging element, such as a driven axle provided with a pair of wheels. The mechanical connection can be a direct connection, such as a drive shaft and a differential or a pair of wheel motors, or an indirect connection, such as a driveline including a transmission or gearbox. In the case of a truck, the first and the second traction motor can drive individual first and second driven axles, or one common driven axle. The first and the second traction motor can also be operated as generators.
The first high voltage circuit and the second high voltage circuit are operated at the same or at similar voltages and are connectable by a third power converter which is located as a bridge between the first and the second high voltage circuits and the first and second power converters. In this context, the term “high voltage” refers to a voltage in a preferred range of 500-800 V. For instance, the first high voltage circuit can be operated at 500-700 V and the second high voltage circuit can be operated at 550-800 V.
The first and the second power converters are preferably DC/AC power converters, or inverters, which are arranged to convert the high voltage direct current to alternating current used for driving the first and second traction motors. The first and second traction motors are preferably three-phase AC motors, which can be synchronous and/or asynchronous, where synchronous motors often use permanent magnets (PMSM). For the purpose of the invention, DC motors can also be used, which DC motors can use brushes or be brushless (BLDC).
The third power converter is a DC/DC power converter. This particular arrangement of the DC/DC power converter is advantageous as it allows the size of the DC/DC power converter to be reduced considerably, relative to a conventional positioning of such a power converter. Examples of relative sizes of DC/DC power converter will be given in the subsequent text. The positioning of the DC/DC power converter also allows for a very flexible use and a number of alternative operating modes, each allowing for a more energy efficient operation and reduced energy losses. Examples of such operating modes are given in the text below.
An advantage is that not all the power from the external power supply needs to pass through the bridge. Instead the main part of the electric power can be directly utilized by the vehicle in the second high voltage circuit. Another advantage is that by splitting the high voltage system, the third converter, or bridge converter, does not need to be at full power range of the propulsion system of the hybrid vehicle. This reduces the size and cost of the third converter. The electrical energy storage system of such a hybrid vehicle may be of any suitable technology, including batteries, super-capacitors, fuel cells and flywheels. By using the energy storage system in the hybrid system, it will be possible to further reduce the required size of the feeding converter for the hybrid system.
The autonomous power supply preferably, but not necessarily, comprises an internal combustion engine connected to the first traction motor. The engine can be used for charging the energy storage system, for instance a battery, by operating the first traction motor as a generator, using the first power converter as a rectifier.
The second high voltage circuit is connectable to an external power supply in the form of overhead wires or a rail. The overhead wires can be accessed through a conventional pantograph or similar, mounted at a suitable location on the vehicle. The rail can be a roadside rail adjacent the route followed by the vehicle, or a recessed rail in the road surface. Examples of such solutions can be found in the “Electric roads concept” proposed by Volvo, or in prior art documents such as WO2012/069495 and CN 102275510, which are incorporated by reference.
As indicated above, the first traction motor and the second traction motor can be connected to individual driven axles, or be connected to a common driven axle. Different operating modes are available depending on the selected mechanical connection for the motors.
A controllable switch can be connected in parallel with the third power converter. The switch is arranged to by-pass the third power converter when closed. Operation of the controllable switch is determined by the operating mode selected, which will be described below.
The invention further relates, according to an aspect thereof, to a method for operating a hybrid energy system in a vehicle provided with an autonomous power supply and being connectable to an external power supply infrastructure along the route of said vehicle.
As indicated above the hybrid energy system comprises a first high voltage circuit comprising a first traction motor for propelling the vehicle connected to an energy storage system by a first power converter, and a second high voltage circuit comprising a second traction motor for propelling the vehicle connectable to an external source of electrical power by a second power converter. The first high voltage circuit and the second high voltage circuit are connectable by a third power converter and by a parallel controllable switch between the first and the second power converters.
The method involves operating the hybrid energy system in any one of a number of alternative modes, which operating modes include at least:                an autonomous power supply mode involving operating the first and second traction motor using the energy storage system;        an external power supply mode involving connecting the third power converter and operating one or both of the first and second traction motors using the external source of electrical power; and        a combined autonomous and external power supply mode involving operating the first traction motor using the energy storage system and the second traction motor using the external source of electrical power.        
In the autonomous power supply mode the energy storage system is used for electric operation of the vehicle, when the external power supply is disconnected. The energy storage system can be used for operating the first traction motor only, using the energy storage system directly via the first power converter.
In the external power supply mode the second traction motor can be connected directly to the external power supply via the second power converter, without losses being incurred in the third power converter. In addition, the external power supply can also be connected to the first traction motor, via the third and the first power converter, order to operate both the first and second traction motors. The energy storage system can be charged from the external power supply during the latter operating mode.
In the combined autonomous and external power supply mode the first traction motor can be operated using the energy storage system via the first power converter, and the second traction motor can be operated using the external source of electrical power, via the first power converter. In this case, the second traction motor can be driven directly by the external power supply, without losses being incurred in the third power converter.
As indicated above, the inventive method allows for a flexible hybrid energy system that can be operated in multiple alternative modes, while minimizing the use of the third power converter. This flexibility is made possible by the location of the third power converter, which is a DC/DC converter. The reduced power requirement for the DC/DC converter allows it to be dimensioned for a relatively small power rating. This in turn allows for a DC/DC converter of smaller size and lower weight, having very high efficiency and reduced heat generation.
According to a further example, the hybrid energy system can be operated in an alternative autonomous power supply mode involving bypassing the third power converter and operating both of the first and second traction motors using the energy storage system. In this example the energy storage system can be used for operating both the first and second traction motor by controlling a switch connected in parallel to bypass the third power converter. The energy storage system can also be used for operating the second traction motor, using the energy storage system directly via the second power converter. In the latter case, the second traction motor can be driven directly by the energy storage system, without losses being incurred in the third power converter. Depending on the design of the vehicle driveline, the first and second traction motors can be used for driving independent first and second driven axles, respectively, or for driving a common driven axle.
According to a further example, the hybrid energy system can be operated in an alternative external power supply mode by bypassing the third power converter. This example involves disconnecting the energy storage system, using existing contactors or circuit breakers connecting the energy storage system to the first high voltage circuit, and operating both the first and second traction motors using the external source of electrical power via their respective power converter. As in the previous example, the first and second traction motors can be used for driving independent first and second driven axles, respectively, or for driving a common driven axle.
Both these alternative operating modes contribute to increased flexibility for the hybrid energy system, by allowing power to be supplied directly to the first and the second traction motor from the on-board energy storage system or the external source of electrical power without incurring losses in the third power converter, which is a DC/DC converter.
The inventive hybrid energy system can also be operated in a number of alternative regenerative operating modes, adding to the flexibility of the system.
According to a further example, the hybrid energy system can be operated in a first alternative regenerative operating mode. In the first alternative regenerative mode the second traction motor is driven using the external source of electrical power to drive a ground engaging element. As described above, the first and the second traction motor can each be mechanically connected to an individual or a common ground engaging element, such as a driven axle provided with a pair of wheels. Accordingly, when the first and the second traction motor are mechanically connected to individual ground engaging elements, the second traction motor can drive the first traction motor indirectly via the ground engaging elements. The second traction motor drives one ground engaging element, whereby a further ground engaging element drives the first traction motor for charging the energy storage system. The first power converter can be used as a rectifier for this purpose.
The first alternative regenerative mode can be used for charging the energy storage system when the third (DC/DC) power converter cannot supply sufficient power for this purpose.
According to a further example, the hybrid energy system can be operated in a second alternative regenerative operating mode. In the second alternative regenerative mode power is supplied to the external source of electrical power by using a controllable switch mounted in parallel to bypass the third power converter and operating one or both of the first and second traction motors as generators using ground engaging elements. As described above, the first and the second traction motor can each be mechanically connected to an individual or a common ground engaging element, such as a driven axle provided with a pair of wheels. The second alternative regenerative mode can be used for braking the vehicle without using the service brakes or when travelling downhill. Kinetic energy is converted to electrical energy by one or both traction motors and is supplied to directly to the external source of electrical power via the respective first and/or second power converters.
The second alternative regenerative mode allows regenerated electrical power to be returned to the grid without using the third (DC/DC) power converter.
According to a further example, the hybrid energy system can be operated in a third alternative regenerative operating mode. In the third alternative regenerative mode power is supplied to the energy storage system by using a controllable switch mounted in parallel to bypass the third (DC/DC) power converter and operating one or both of the first and second traction motors as generators using ground engaging elements. As described above, the first and the second traction motor can each be mechanically connected to an individual or a common ground engaging element, such as a driven axle provided with a pair of wheels. The second alternative regenerative mode can be used for braking the vehicle without using the service brakes or when travelling downhill instead of using compression braking. Kinetic energy is converted to electrical energy by one or both traction motors and is supplied directly to the energy storage system via the respective first and/or second power converters. During this operation, the external power supply must be disconnected.
The third alternative regenerative mode allows regenerated electrical power to be returned to the energy storage system without using the third (DC/DC) power converter.
According to a further example, the hybrid energy system can be operated in a fourth alternative regenerative operating mode. In the fourth alternative regenerative operating mode the second traction motor is driven using the external source of electrical power. When the first and the second traction motor are mechanically connected to a common ground engaging element, the second traction motor can drive the first traction motor directly via a mechanical connection in the transmission for charging the energy storage system. This involves disconnecting both traction motors from the part of the vehicle transmission connecting them to the ground engaging elements. The first traction motor is then driven using the second traction motor to charge the energy storage system.
The fourth alternative regenerative mode can be used for charging the energy storage system when the vehicle is standing still, without using the third (DC/DC) power converter.
The invention further relates, according to an aspect thereof, to a vehicle, preferably but not necessarily a commercial vehicle, comprising a hybrid energy system as described and operated according to the above text.
The present invention also relates to a computer program, computer program product and a storage medium for a computer all to be used with a computer for executing the method as described in any one of the above examples.
A hybrid energy system as described above, comprises two high voltage circuits which are connected by a DC/DC converter and can be operated at different tolerance levels. For instance, in an exemplary system, the nominal voltage in the two electrical circuits can be e.g. 650 V while the actual voltage in a first of the circuits can vary between 500 V and 900 V. If the allowable variation in the second circuit is limited to 550 V to 800 V, then a DC/DC converter can be connected between the two circuits to allow power transfer without running the risk of interference or damage to the system.
Further advantages with the arrangement are that fewer components require to be galvanically isolation from the vehicle chassis. According to the invention it is sufficient to provide galvanic isolation for the DC/DC converter, the second electric motor and its inverter (second power converter). A conventional system, e.g. as described in FIG. 4, would require galvanic isolation for the entire system including the first electric motors and the energy storage system. By providing a power converter in the form of a DC/DC converter connecting the two high voltage circuits the system can be operated in multiple different modes without requiring all electric power to pass through the DC/DC converter. This results in reduced conversion losses and heat generation requiring cooling, which in turn improves the overall system efficiency and reduces the demand on the vehicle cooling system. The power rating of the DC/DC converter can also be reduced, which allows the size of the converter to be reduced and contributes to a more compact installation.